Low power switched mode power supply

Abstract

A switched mode power supply for supplying power from a power source to one or more output voltage rails, including: a switching unit configured to receive a source voltage from the power source and to output a switched voltage, the switching unit including: a first switch configured to switch the source voltage to the switched voltage, and configured to be driven by the source voltage; and a second switch connected in parallel with the first switch, the second switch configured to switch the source voltage to the switched voltage, and configured to be driven by an output voltage of one of the output voltage rails.

The invention claimed is: 
     
       1. A switched mode power supply for supplying power from a power source to one or more output voltage rails, comprising:
 a switching unit configured to receive a source voltage from the power source and to output a switched voltage, the switching unit comprising: 
 a first switch configured to switch the source voltage to the switched voltage, and configured to be driven by the source voltage; and 
 a second switch connected in parallel with the first switch, the second switch configured to switch the source voltage to the switched voltage, and configured to be driven by an output voltage of one of the output voltage rails. 
 
     
     
       2. A switched mode power supply as claimed in  claim 1 , wherein the first and second switches are FETs. 
     
     
       3. A switched mode power supply as claimed in  claim 2 , wherein one of the first and second switches is a PMOS transistor and the other of the first and second switches is an NMOS transistor. 
     
     
       4. A switched mode power supply as claimed in  claim 3 , wherein the first switch is a PMOS transistor, and the second switch is an NMOS transistor. 
     
     
       5. A switched mode power supply as claimed in  claim 3 , wherein the first switch is an NMOS transistor, and the second switch is a PMOS transistor. 
     
     
       6. A switched mode power supply as claimed in  claim 1 , wherein the switching unit further comprises:
 a first driver connected to the first switch, the first driver configured to convert the source voltage to a first drive voltage for driving the first switch; and 
 a second driver connected to the second switch, the second driver configured to convert the output voltage to a second drive voltage for driving the second switch. 
 
     
     
       7. A switched mode power supply as claimed in  claim 6 , further comprising a controller configured to control the first driver and the second driver to drive the first and second switches such that both switches switch the source voltage to the switched voltage contemporaneously. 
     
     
       8. A method of operating a switched mode power supply for supplying power from a power source to one or more output voltage rails, the switched mode power supply comprising a switching unit that receives a source voltage from the power source and outputs a switched voltage, the switching unit comprising a first switch and a second switch connected in parallel, the method comprising:
 driving the first switch by the source voltage to switch the source voltage to the switched voltage; and 
 driving the second switch by an output voltage of one of the output voltage rails to switch the source voltage to the switched voltage. 
 
     
     
       9. A method as claimed in  claim 8 , wherein the switching unit comprises a first driver connected to the first switch and a second driver connected to the second switch, the method comprising:
 driving the first switch by, at the first driver, converting the source voltage to a first drive voltage for driving the first switch; and 
 driving the second switch by, at the second driver, converting the output voltage to a second drive voltage for driving the second switch. 
 
     
     
       10. A method as claimed in  claim 9 , further comprising controlling the first driver and the second driver to drive the first and second switches such that both switches switch the source voltage to the switched voltage contemporaneously. 
     
     
       11. A method as claimed in  claim 8 , wherein the output voltage is higher than the source voltage. 
     
     
       12. A method as claimed in  claim 8 , wherein the first switch is a FET and the output voltage is higher than the combination of the source voltage and a gate-source threshold voltage of the FET. 
     
     
       13. A method as claimed in  claim 8 , comprising only driving the second switch by the output voltage to switch the source voltage to the switched voltage if the source voltage is less than a threshold voltage. 
     
     
       14. A method as claimed in  claim 13 , wherein the first switch is a FET and the threshold voltage is a gate-source threshold voltage of the FET. 
     
     
       15. A method as claimed in  claim 8 , wherein the switched mode power supply operates in a first mode when the source voltage is below a threshold voltage and operates in a second mode when the source voltage is above the threshold voltage. 
     
     
       16. A method as claimed in  claim 15 , wherein in the first mode, the method comprises driving the second switch by converting the output voltage to a second drive voltage for driving the second switch. 
     
     
       17. A method as claimed in  claim 15 , wherein in the second mode, the method comprises disabling the driving of the second switch. 
     
     
       18. A method as claimed in  claim 17 , wherein the driving of the second switch is disabled by disabling the supply of current to the output voltage rail having the output voltage.